Sentry Key Immobilizer System
The Sentry Key Immobilizer System (SKIS) is designed to provide passive protection against unauthorized vehicle use by disabling the engine after about two seconds of running when any method other than a valid Sentry Key is used to start the vehicle. The SKIS is considered a passive protection system because it is always active when the ignition system is energized and does not require any customer intervention. The functions and features of the SKIS are performed by hardware and software integral to the Body Control Module (BCM). The BCM contains a Radio Frequency (RF) transceiver and a microcontroller. The BCM transmits and receives LF signals from the Sentry Key transponder integral to the Sentry Key transponder through a tuned antenna external to the ignition switch lock housing. The BCM communicates over the Controller Area Network (CAN) data bus with the Instrument Panel Cluster (IPC), Powertrain Control Module (PCM), or the diagnostic scan tool.
The BCM and the PCM both use software that includes a rolling code algorithm strategy, which helps to reduce the possibility of unauthorized SKIS disarming. The rolling code algorithm ensures security by preventing the SKIS from overriding through the unauthorized substitution of the BCM or the PCM. However, this strategy's use also means that the replacement of either the BCM or the PCM units will require a system initialization procedure to restore system operation.
The BCM retains in memory the ID numbers of any Sentry Key transponder that is programmed into it. A maximum of eight Sentry Key transponders can be programmed into the BCM. For added system security, each BCM is programmed with a unique Secret Key code. This code is stored in memory, sent over the CAN data bus to the PCM, and encoded to the transponder of every Sentry Key programmed into the BCM. Therefore, the Secret Key code is a common element found in every component of the SKIS.
If a BCM replacement is required, the new BCM must be obtained through an authorized Service Center. The Service Center will program and initialize the replacement BCM before installation in the vehicle. Proper completion of the BCM initialization will allow the existing Sentry Key transponders to be programmed into the new BCM so that new Sentry Key transponders will not be required.
When the status of the ignition switch transitions to On, the BCM transmits an LF signal to excite the transponder in the Sentry Key. The BCM then waits for an LF signal response from the transponder. If the response received identifies the transponder as valid, the BCM sends an electronic valid key message to the PCM over the CAN data bus. If the response received identifies the transponder as invalid or if no response is received from the transponder, the BCM sends an invalid key message to the PCM. The PCM will enable or disable engine operation based upon the status of the BCM messages. It is important to note that the default condition in the PCM is an invalid key ; therefore, if no message is received from the BCM by the PCM, the engine will be disabled and the vehicle immobilized after two seconds of running.
The BCM also sends electronic security indicator request messages to the IPC over the CAN data bus to tell the IPC how to operate the security indicator. The security indicator request message from the BCM tells the IPC to turn the indicator On for about three seconds each time the status of the ignition switch transitions to On as a bulb test. After completing the bulb test, the BCM sends a security indicator request messages to the IPC to turn the indicator Off, turn the indicator On, or flash the indicator On and Off. If the security indicator flashes or stays On solid after the bulb test, it signifies a SKIS fault. If the BCM detects a system malfunction or the SKIS has become ineffective, the security indicator will stay On solid. If the BCM detects an invalid Sentry Key transponder or if a transponder-related fault exists, the security indicator will flash.
The SKIS performs a self-test each time the status of the ignition switch transitions to On and will store fault information in the form of a Diagnostic Trouble Code (DTC) in BCM memory if a system malfunction is detected. The hardwired circuits of the BCM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in diagnosing the BCM or the electronic controls and communication between other modules and devices that provide some features of the SKIS. The most reliable, efficient, and accurate means to diagnose the BCM or the electronic controls and communication related to SKIS operation requires the use of a diagnostic scan tool.